Block-based motion-predictive video coding is one of the most commonly used techniques for video compression. Standards such as MPEG-2, MPEG-4, Windows Media Video (“WMV”) versions 7/8/9, VC-1, and H.264, are based on block-based motion-video predictive coding techniques. These video compression techniques typically encode individual frames of video using intraframe compression or interframe compression. Intraframe compression techniques compress an individual frame without reference to video data from other frames. Such frames are typically referred to as “I-frames” or “key frames.” Interframe compression techniques compress frames with reference to preceding frames or with reference to both preceding and subsequent frames. Frames compressed with reference to preceding frames are typically referred to as “predicted frames” or “P-frames,” whereas frames compressed with reference to both preceding and subsequent frames are referred to as “B-frames” or “bidirectionally predicted frames.”
The common detriment of block-based techniques is the creation of artificial illusory boundaries, or contours, between blocks in the decompressed video. These contours are referred to as “block artifacts,” “blocking artifacts,” or “blockiness.” Blockiness is generally worse when the video bit rate is lower and is highly undesirable.
Many techniques have been proposed to reduce block artifacts, including overlapped motion compensation, wavelets, large-support transforms, and deblocking filters. Of these, only deblocking filters have been found to be useful and effective for practical and commercial video encoders. A deblocking filter in video coding is a filter that smoothes out block boundaries in a decompressed video frame using a set of rules that is implicitly derived from data known to the decoder. In other words, deblocking filters generally require no additional information to be sent in or with the compressed video stream.
Furthermore, deblocking filters are sometimes used selectively or varied in strength depending on the particular use. For example, the strength of the deblocking filter used in the H.264/AVC standard is adjusted based on the block types (intra or inter) and on whether the blocks contain coded coefficients. Furthermore, the H.264/AVC deblocking filter is disabled for a block when the intensity difference between boundary pixels in two adjacent blocks indicates that there is a significant change across the block boundary. Such a significant change indicates that the block edge is more likely to be an original image feature rather than a blocking distortion caused by a block artifact. For the H.264/AVC filter, the thresholds defining whether a significant change is present depend on the quantization parameter (“QP”).
Although the deblocking scheme described in the H.264/AVC standard helps reduce some blocking artifacts, the scheme suffers from a number of disadvantages. First, deblocking is always performed along the direction perpendicular to the block boundary (either horizontally or vertically), without any consideration for local features inside the block. Such a scheme is therefore not equipped to account for and properly filter image edges that are oriented in non-horizontal and non-vertical orientations, which can be the large majority of edges in an image. Further, the same filter orientation and thresholds are applied to highly different image content, which can potentially degrade the deblocking performance.
Additionally, large smooth regions in video images can be the source for significant blocking artifacts. In particular, for high resolution videos compressed at a low bit-rate, the most visually annoying artifacts sometimes appear in a large smooth region that has only a small color or intensity variation across the region. Although the blocking degradation in such regions is reduced to a certain extent using standard deblocking schemes (such as the H.264/AVC deblocking scheme), significant blockiness still visibly exists spatially and temporally. This blocking is often highly apparent and can heavily impair the perceptual experience of the audience.